1. Field of the Invention
The invention relates generally to catalytic combustion systems and control methods, and more particularly to systems and methods for detecting and responding to catalyst module overheating or conditions that may result in a catalyst module overheating in single or multi-combustor processes as they relate to and are utilized by catalytic gas turbine engines.
2. Description of the Related Art
In a conventional gas turbine engine, the engine is controlled by monitoring the speed of the engine and adding a proper amount of fuel to control the engine speed. Specifically, should the engine speed decrease, fuel flow is increased causing the engine speed to increase. Similarly, should the engine speed increase, fuel flow is decreased causing the engine speed to decrease. In this case, the engine speed is the control variable or process variable monitored for control. A similar engine control strategy is used when the gas turbine is connected to an AC electrical grid in which the engine speed is held constant as a result of the coupling of the generator to the grid frequency. In such a case, the total fuel flow to the engine may be controlled to provide a given power output level or to run to maximum power with such control based on controlling exhaust gas temperature or turbine inlet temperature. Again, as the control variable rises above a set point, the fuel is decreased. Alternatively, as the control variable drops below the set point, the fuel flow is increased. This control strategy is essentially a feedback control strategy with the fuel control valve varied based on the value of a control or process variable compared to a set point.
In a typical non-catalytic combustion system using a diffusion flame burner or a simple lean premixed burner, the combustor has only one fuel injector. In such systems, a single valve is typically used to control the fuel flow to the engine. In more recent lean premix systems however, there may be two or more fuel flows to different parts of the combustor, with such a system thus having two or more control valves. In such systems, closed loop control may be based on controlling the total fuel flow based on the required power output of the gas turbine while fixed (pre-calculated) percentages of fuel flow are diverted to the various parts of the combustor. The total fuel flow will change over time. In addition, the desired fuel split percentages between the various fuel pathways (leading to various parts of the combustor) may either be a function of certain input variables or they may be based on a calculation algorithm using process inputs such as temperatures, airflow, pressures, and the like. Such control systems offer ease of control due primarily to the very wide operating ranges of these conventional combustors and the ability of the turbine to withstand short spikes of high temperature without damage to various turbine components. Moreover, the fuel/air ratio fed to these combustors may advantageously vary over a wide range with the combustor remaining operational.
A properly operated catalyst combustion system may provide significantly reduced emission levels, particularly NOx. Unfortunately, however, such systems generally have a narrower operating range than conventional diffusion flame or lean premix combustors. For example, operation of catalytic combustors above desired temperature limits, which may vary depending on the particular application and design of each combustor system, may result in thermal damage to the catalytic module. Such operation could be a result of a variety of contributing factors in single or multi-combustor applications, including variations in fuel composition outside of specifications, blockage of catalytic channels caused by foreign objects, lack of uniformity of inlet fuel-air mixture, flameholding in the burn-out-zone radiating heat back to the catalyst, or lack of uniformity of catalytic material on the substrate due to manufacturing variability. In multi-combustor specific applications discussed below, operation above desired temperature limits may be a result of combustor-to-combustor non-uniformities.
The configuration of industrial gas turbines with conventional, non-catalytic combustors, varies from simple single-silo configurations, i.e., one combustor as discussed above, to multiple-combustor configurations. The application of industrial, or otherwise, gas turbine engines with catalytic combustion, however, has been limited to the single-silo configuration. For example, the Kawasaki Heavy Industries M1A-13X and the GE 10 (PGT 10B) gas turbine engines.
The application of catalytic combustion in a multi-combustor configuration poses several additional problems that may lead to thermal damage to one or more catalyst modules. For example, in a multi-combustor configuration there typically are variations from combustor-to-combustor due to manufacturing or design differences that may lead to variations in pre-burner ignition, catalyst light-off, and/or homogeneous combustion in the burnout zone across the multiple combustors. Additionally, the combustor sizes are typically reduced to prevent combustor-to-combustor physical interference adding complexity to the design of the combustors. Combustor size reduction can be achieved through flame-holders in the burn-out zone and single-stage catalyst designs. To supplement the single stage catalyst designs, pre-burners with wide turn-down ratios are generally used. These design changes will require more complex control of the pre-burner, catalyst fuel/air ratio, and/or post catalyst homogenous combustion burnout zone to ensure the combustion system operates within its operating window.
What is needed therefore is a method and system for detecting catalyst module overheating or conditions that may result in catalyst module overheating in single and multi-combustor systems. Additionally, methods of controlling catalytic combustion systems including single and multiple combustors in response to catalyst module overheating is needed. Finally, a method and system are needed that reduce the potential for overshooting and exceeding desired temperature limits for the catalyst module during transient operations, such as load ramps and the like.